1. Field of the Invention
Generally, the invention relates to testing storage devices under certain test conditions. More specifically, the invention relates to adjusting an error correction capability of an LDPC code such that the sector failure rate of the storage device can be incrementally determined.
2. Discussion of the Related Art
In many storage devices, data is commonly written to and retrieved from the devices in contiguous blocks. In a disk drive, a read/write channel performs such reading and writing. The read/write channel improves the detection of the data that is written to and read from the hard disk using certain detection algorithms. For example, algorithms like PRML (Partial Response signaling with Maximum Likelihood detection) increase the signal to noise ratio (SNR) of the data being read from the hard disk which increases the likelihood that the data will be read properly. Since correct detection of the data is more likely, these algorithms may allow for the increase of areal densities on the disk (i.e., an increased number of storable bits on the hard disk). As data errors can increase with increased areal density, however, error control coding is used to improve the performance of the detection algorithm. Often, Reed-Solomon codes (RS codes) provide for this error correction capability.
The number of data errors that occurs during a read (i.e., the sector failure rate, or SFR) generally coincides with the SNR of the read signal. Lower SNRs result in greater error rates. Higher SNRs result in lower error rates. The SNR of the read signal has certain limits due to power, componetry, and degradation of the disk drive over time. The data errors that result are thus corrected with the RS code.
Disk drive manufacturers generally establish a code level of “T=20” for the maximum level of correction required to meet the maximum error rates that a disk drive is expected to endure as the hard disk degrades. A T=20 code level requires 20 bits of code for 40 bits of data. During the manufacturing process, the disk drives are tested to ensure that the drives meet some requisite level of error rates for given SNRs. Since the hard disks are new, they are likely to encounter fewer errors than their aging counterparts. However, some new disks will experience an unacceptable number of errors due to inconsistencies in the manufacturing process. To ensure that the disks meet acceptable SFRs for certain SNRs without being recovered by the full T=20 error correction, the code level is “dialed back” to T=5.
Presently, disk drive manufacturers are seeking to employ other codes as areal densities and processing capabilities increase with technological advances. One code presently being implemented is called the Low Density Parity Check (LDPC) code. The LDPC codes are relatively complex and thus computationally intensive. For example, the LDPC codes correct errors in larger contiguous blocks of data than the RS codes and thus require more computations. The LDPC codes are generally capable of correcting more errors than the RS codes and appear to be the heirs apparent in error correction as processing capabilities have significantly increased over time. The LDPC codes, however, do not have the RS code capability of “dialing back” to ensure that manufactured test drives meet some requisite minimum level of SFRs. Thus, there exists a need to reduce the level of LDPC error correction for testing purposes if the LDPC codes are to be used in storage devices.